1. Field of the Invention
The present invention relates generally to a method for processing a feedback signal and a pre-distorter in a mobile communication system. More particularly, the present invention relates to a method and apparatus for stably and closely estimating a sample delay between a transmission signal and a feedback signal output from various boards of a mobile communication system regardless of a frequency allocation (FA) value. Moreover, the present invention relates to an apparatus and method for improving performance of a pre-distorter required to closely estimate a sample delay between a transmission signal and a feedback signal in a mobile communication system.
2. Description of the Related Art
Conventionally, desired signal processing is performed using a transmission signal which is to be processed and output by a designated operation routine in a board mounted in a communication device such as a base station or the like, and a feedback signal associated therewith. Delay essentially occurs according to a signal path between the transmission signal and the feedback signal. To stably process a signal, a task for estimating a signal delay and performing synchronization between the transmission and feedback signals is important.
A signal delay estimation method for performing synchronization between the transmission and feedback signals in the mobile communication system is referred to as a sample delay estimation method. For example, a conventional sample delay estimation method stores a plurality of samples of the transmission and feedback signals, performs a correlation operation on the stored samples, and estimates a sample delay interval with the largest correlation value. A method for estimating the sample delay interval is conventionally referred to as rough delay estimation.
Furthermore, the mobile communication system performs fine delay estimation for estimating a sample delay interval using the conventional rough delay estimation method using the largest correlation value, performing time synchronization between the transmission and feedback signals, and finely dividing the estimated sample delay interval, for example, in a 1/10 sample interval. Moreover, the mobile communication system performs desired signal processing based on signal synchronization between the transmission and feedback signals using the sample delay interval estimated by the fine delay estimation.
A signal processor is required to estimate the sample delay interval between the transmission and feedback signals and is comprised of a digital pre-distorter provided in a base station transmitter or other location. A power amplifier is also required for amplifying the transmission signal in the base station transmitter. The power amplifier has non-linear operation characteristics.
Because a high power amplifier (HPA) which is widely used to obtain high efficiency in the base station transmitter of the mobile communication system operates close to a non-linear operation point, the power amplifier generates an inter-modulation distortion (IMD) component. The IMD component affects other frequency bands, as well as in-band. Accordingly, the pre-distorter generates a pre-distortion signal having the same signal size as that of the IMD component generated from the power amplifier and having a phase that is opposite to the phase of the IMD component. The pre-distorter compensates an output signal of the power amplifier using the pre-distortion signal. Accordingly, the output signal of the power amplifier has linear characteristics.
The pre-distorter receives the output signal of the power amplifier for signal compensation as a feedback signal, and performs a digital pre-distortion (DPD) algorithm for eliminating a spurious component. Samples of both the transmission and feedback signals must be stored during a previous phase, and a sample delay interval must be estimated using the stored samples, such that the DPD algorithm can be performed.
However, the conventional sample delay estimation method has a problem in that an error of the estimated sample delay interval occurs according to a frequency allocation (FA) value of the transmission and feedback signals serving as input signals. For example, in a code division multiple access (CDMA) 2000 1x system, a sample delay interval is closely estimated from 12-FA input signals in a block with a size of 1,024 samples. However, a significant error in the sample delay interval estimated from the input signals with a small FA value also occurs in the block with a size of 1,024 samples.
FIGS. 1A through 1C are waveform diagrams illustrating simulation results according to a conventional sample delay estimation method, and illustrate the results of the conventional rough delay estimation for storing a plurality of samples of transmission and feedback signals as input signals, performing the maximum correlation operation on the stored samples, and estimating the sample delay interval.
First, FIG. 1A illustrates simulation results obtained by estimating a sample delay interval from 12-FA CDMA 2000 1x input signals in a unit of 1,024 samples. It can be found that a sample delay interval regularly converges, for example, in an 18-sample interval. Next, FIG. 1B illustrates simulation results obtained by estimating a sample delay interval from 4-FA CDMA 2000 1x input signals in the unit of 1,024 samples. The FA value of FIG. 1B is less than that of FIG. 1A. As illustrated in FIG. 1B, it can be found that an error in a sample delay interval estimated between a 17-sample interval and a 19-sample interval occurs. The waveforms (A), (B), (C), and (D) of FIG. 1C represent accumulated simulation results obtained by repeating a rough delay estimation operation on 1-FA CDMA 2000 1x input signals 1, 6, 8, and 14 times. When the accumulated number of times increases, it can be found that an error in an estimated delay interval is gradually reduced.
Since an estimation error in a sample delay interval is large when an FA value is small in the case of the conventional rough delay estimation, it is difficult for a sample delay to be closely estimated. Also, since the sample block length or the number of estimation operations must be increased such that an estimation error can be reduced in the case of a system with a small FA value, there is a problem in that system resources are wasted.
Accordingly, a need exists for a method and apparatus that can closely estimate a sample delay between a transmission signal and a feedback signal regardless of a frequency allocation (FA) value by using minimal system resources.